1. Field of the Invention
The present invention relates to a method for removing hydrogen chloride from a flue gas.
2. Description of the Related Art
A device for treating of a flue gas containing hydrogen chloride is installed, for example, in a waste treatment equipment of subjecting waste to a incineration treatment. In this waste treatment equipment, a first dust collector and a second dust collector are arranged in series, and after removal of dust such as combustion fly ash contained in flue gas by the first dust collector, a dechlorination of flue gas is carried out in the second dust collector.
To carry out dechlorinating in the second dust collector, a dechlorinating agent is added into flue gas before the second dust collector. As a dechlorinating agent, a calcium-based dechlorinating agent such as calcium hydroxide (Ca(OH)2) has conventionally been mainly employed. Calcium hydroxide, if added into flue gas, reacts with hydrogen chloride (HCl) contained in the flue gas to generate residue of dechlorination containing calcium chloride (CaCl2), calcium oxide (CaO) and the like. However, thus generated residue of dechlorination is useful only in that calcium chloride is applied as a snow melting agent or a moisture absorbent, with a narrow range of effective uses. Residue of dechlorination is mostly solidified through a chemical treatment or with cement and disposed of in reclamation. However, acquisition of reclamation site is now becoming more difficult.
It is therefore proposed to use a sodium-based dechlorinating agent such as sodium hydrogencarbonate (sodium bicarbonate: NaHCO3) or sodium carbonate (soda ash: Na2CO3) in place of the calcium-based dechlorinating agent. In this case, when a sodium-based dechlorinating agent is added into a flue gas, hydrogen chloride contained in the flue gas becomes sodium chloride (NaCl). Adding water to residue of dechlorination dissolves sodium chloride. Therefore, water-soluble constituents dissolved in water are diluted and discharged, and only water-insoluble constituents not dissolved in water are separated and can be subjected to a combustion treatment in a melting furnace, thus eliminating the necessity of disposal in reclamation. When sodium hydrogencarbonate is adopted as a sodium-based dechlorinating agent and sodium hydrogencarbonate has a particle size larger than 30 xcexcm, powder particles never coagulating together between them, and the agent is stable as powder. However, sodium hydrogencarbonate having a particle size larger than 30 xcexcm leads to a very low removing ratio of hydrogen chloride, so that the use thereof as a dechlorinating agent is not appropriate. In general, therefore, sodium hydrogencarbonate ground to a particle size of 30 xcexcm or less is used as a sodium-based dechlorinating agent.
However, when sodium hydrogencarbonate is ground to a particle size of 30 xcexcm or less, coagulation of powder particles results in a form of fibrous dust balls or stone-like lumps. Ground sodium hydrogencarbonate has thus an unstable condition as powder, thus making is impossible to stably supply the same to flue gas.
To solve this defect, it is the usual practice to use an anti-caking agent. In the conventional art, a hydrophobic anti-caking agent has been used as such an anti-caking agent. A hydrophobic anti-caking agent brings about a remarkable solidification inhibiting effect. Sodium hydrogencarbonate added with a hydrophobic anti-caking agent has high flowability and floodability property, and exhibits satisfactory stability as a powder.
However, when a hydrophobic anti-caking agent added with sodium hydrogencarbonate is added into a flue gas as a dechlorinating agent, particles resulting from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, and particles of the anti-caking agent, which both have a high flowability, tend to easily entrap inside a filter cloth attached in a second dust collector comprising, for example, a bag filter. When particles of residue of neutralization or particles of the anti-caking agent penetrate into the filter cloth, there is caused clogging, resulting in an excessive pressure drop at the filter cloth and making it impossible to continue operation. It is difficult to recover from clogging even by back washing of the bag filter by the use of pulse air.
Further, some of particles produced from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, having entrapped into the filter cloth and particles of the anti-caking agent pass through the filter cloth, causing leakage of the dechlorinating agent and residue of neutralization. A double-woven glass cloth is usually used as a filter cloth. In order to prevent leakage of the dechlorinating agent and residue of neutralization, it is necessary to use a special filter cloth made by applying a Teflon membrane coated to the surface of the double-woven glass cloth. If the membrane is damaged or peeled off during use, however, a new problem of leakage of chemicals from this portion is encountered.
An object of the present invention is to prevent occurrence of an excessive pressure drop or leakage in the filter cloth attached to the dust collector.
For the purpose of solving the aforementioned problems, improvement is made for the sodium-based dechlorinating agent in the present invention. More particularly, the sodium-based dechlorinating agent of the invention comprises a mixture of sodium hydrogencarbonate and a hydrophilic anti-caking agent, and has an angle of repose of 40xc2x0 or more, a dispersibility of less than 50, and a floodability index of less than 90.
According to the sodium-based dechlorinating agent having the above-mentioned configuration, in which the hydrophilic anti-caking agent has a slight cohesion, flowability of sodium hydrogencarbonate particles and the anti-caking agent becomes sluggish: sodium hydrogencarbonate particles or anti-caking agent particles never come in the filter cloth, and form a stable filtration layer on the surface of the filter cloth. It is consequently possible to prevent occurrence of an excess pressure drop in the filter cloth, and occurrence of leakage from the filter cloth.
Physical properties were measured using a powder tester Model PT-D made by Hosokawa Micron Corporation.
The measurement of the angle of repose is carried out, by causing a powdery sample to pass through a screen having a diameter of 80 mm and a mesh opening of 710 xcexcm while vibrating the screen, and gently dropping the sieved sample from a funnel having a height of 160 mm onto a horizontal table having a diameter of 80 mm, as an angle formed between a generating line of a cone formed by the powder and the horizontal plane, and takes a smaller value according as flowability is higher. The falling amount of powder is measured until the angle of repose becomes substantially stabilized.
The floodability index value is a criterion for numerically evaluating the flood property. The floodability index value is defined, by determining indices from Tables 5 and 6 from measured values of flowability index value, angle of fall, angle of difference and dispersibility, as a value available by summing up these indices: a larger value corresponds to a higher floodability property. The definitions of the individual physical properties will now be described. The flowability index value is determined by similarly determining indices from measured values of angle of repose, compressibility, angle of spatula and uniformity coefficient, and expressed in a value obtained by summing up these values of indices. The angle of repose is determined by the above-mentioned method.
The compressibility is defined as:
{(Packed bulk density)xe2x88x92(Aerated bulk density)}/ (Packed bulk density)xc3x97100
The aerated bulk density is determined, by causing a powdery sample to pass through a screen having a diameter of 80 mm and a mesh opening of 710 xcexcm while vibrating the same, and filling a container having an inner volume of 100 cm3 just with the powder under screen, as a measured mass of the powder. The packed bulk density is determined by causing a container containing powder to tap at a pace of 180 taps for a period of 180 seconds, and measuring the mass corresponding to a volume of 100 cm3. The angle of spatula is determined by keeping horizontally a spatula made of a metal having a size 120xc3x9722 mm, and measuring the inclination angle of the side surface of the powder accumulated thereon.
The uniformity is defined, in the cumulative mass distribution obtained from a particle size distribution measured by sieve analysis, as a value obtained by dividing the particle diameter at 60% of cumulative undersize distribution by the particle diameter at 10% cumulative undersize distribution. The particle size distribution is determined by any of various methods such as the sieve analysis and the laser diffraction scattering method, in response to the particle size of the powder to be measured and the like. Values measured by the laser diffraction scattering method are adopted in the invention, using a xe2x80x9cMicrotrack FRA9220xe2x80x9d made by Nikkiso Co., Ltd. for measurement.
The angle of fall is determined by applying a constant impact force three times with a shocker annexed to the measuring instrument to the cone of powder formed for the purpose of measuring the angle of repose, and measuring the inclination angle of a cone formed as a result of fall.
The angle of difference is determined as a value obtained by subtracting the value of angle of fall from the value of angle of repose.
The dispersibility is determined by dropping 10 g powder sample from a height of 61 cm at a stroke onto a watch glass having a diameter of 10 cm installed with the concave side down, as a percentage of the mass of the powder sample scattering outside the watch glass relative to the total mass of the dropped powder sample. A powder sample higher in this value has generally high scattering property and floodability.
Sodium hydrogencarbonate should preferably have a mean particle diameter within a range of from 2 xcexcm to 30 xcexcm. With the mean particle diameter larger than 30 xcexcm, reaction between the sodium-based dechlorinating agent and hydrogen chloride may become insufficient. With the mean particle diameter smaller than 2 xcexcm, on the other hand, much time and labor may be required for grinding. In order to achieve a sufficient reaction of the sodium-based dechlorinating agent, a mean particle diameter suffices to be within a range of from 2 xcexcm to 30 xcexcm, a mean particle diameter within a range of from 2 xcexcm to 10 xcexcm gives a higher reaction, and is therefore more recommendable.
Applicable anti-caking agents generally include silica, magnesium stearate, calcium stearate and magnesium carbonate.
The hydrophilic anti-caking agent in the invention should preferably comprise silica and should preferably be mixed in the total mass of the dechlorinating agent in an amount of 0.1% or more. An amount less than 0.1% is insufficient to obtain an anti-caking effect. Since the hydrophilic anti-caking agent is insoluble in water, a high blending ratio of agent would require much time and labor for treating residue of dechlorination. The hydrophilic anti-caking agent should more preferably be mixed therefore in the total mass of the dechlorinating agent in an amount within a range of from 0.1% to 5%.
Further, a mean particle diameter of the hydrophilic anti-caking agent should preferably be within a range of from 0.001 xcexcm to 1 xcexcm. While a smaller particle diameter leads to a more remarkable anti-caking effect, it is not technically easy to achieve a powder of a particle diameter smaller than 0.001 xcexcm industrially at a low cost. A mean particle diameter larger than 1 xcexcm results in a smaller effect of anti-caking. It should therefore be more preferably within a range of from 0.001 xcexcm to 0.1 xcexcm.
The waste treatment equipment of the present invention comprises a pyrolytic reactor which causes pyrolysis of waste to generate pyrolytic gases and pyrolytic residue mainly comprising non-volatile constituents; separating means for separating the pyrolytic residue into combustible constituents and incombustible constituents; a combustion melting furnace to which the pyrolytic gases and the combustible constituents are fed, and which causes combustion thereof and discharges molten slag and flue gases; first flue gas treating means for removing dust from the flue gases; second flue gas treating means dechlorinating the flue gases from the first flue gas treating means by adding a dechlorinating agent; a separator which separates water-insoluble constituents not dissolved in water from an aqueous solution containing the residue of dechlorination dissolved therein by adding water to the residue of dechlorination generated by the second flue gas treating means; a pH modifier which adjusts pH of the remaining aqueous solution after separation by the water-insoluble constituents by the separator; and at least another one dioxin removing unit which removes dioxin and the like from the residue of dechlorination generated by the second flue gas treating means and/or from the aqueous solution of which pH has been adjusted by the pH modifier; wherein a sodium-based dechlorinating agent is added to the second flue gas treating means.
As described above, a dioxin removing unit for removing dioxin and the like from the aqueous solution of which pH has been adjusted by the pH modifier may be installed as a dioxin removing unit. In place of this dioxin removing unit for removing dioxin from the aqueous solution of which pH has been adjusted, a dioxin removing unit for removing dioxin and the like from residue of dechlorination generated by the second flue gas treating means may be provided. When the dioxin removing unit for removing dioxin and the like from residue of dechlorination generated by the second flue gas treating means is provided, this dioxin removing unit can remove dioxin and the like almost totally, but in order to remove dioxin and the like not removed, a second dioxin removing unit may be provided in the downstream of the pH modifier.
It is possible to remove mercury from the aqueous solution remaining after separation of water-insoluble constituents by the use of a chelating resin or a chelating agent (hereinafter collectively referred to as xe2x80x9cchelating substancexe2x80x9d).
Further, a mixer for mixing sodium hydrogencarbonate and the hydrophilic anti-caking agent, and a grinder for grinding sodium hydrogencarbonate may be provided in the upstream of the second flue gas treating means in the aforementioned waste treatment equipment. A mercury removing unit for removing mercury from the aqueous solution of which pH has been adjusted by the pH modifier may be added in the downstream of the pH modifier in the aforementioned waste treatment equipment.
The flue gas dechlorinating method of the invention comprises the step of adding the above-mentioned sodium-based dechlorinating agent into the flue gases to cause hydrogen chloride contained in the flue gases to react with the sodium-based dechlorinating agent to remove the same.
Further, the flue gas dechlorinating method of the invention comprises the steps of causing hydrogen chloride contained in the flue gas to react with the sodium-based dechlorinating agent to remove the same as residue of dechlorination, removing dioxin and the like from the residue of dechlorination, then, dissolving the residue of dechlorination by adding water, separating water-insoluble constituents not dissolved in water from an aqueous solution in which the residue of dechlorination is dissolved, and adjusting pH of the remaining aqueous solution after separation of the water-insoluble constituents.
In the above-mentioned flue gas dechlorinating method, dioxin and the like not fully removed may be removed again after pH adjustment.